The Manhattan Project was the codename for a project conducted during World War II to develop the first atomic bomb. The Manhattan Project startd in the 1930's because the United States was scared that Germany would start using neclear weapons. It wasn't even a form of offense to start with but a form of defence and it ended up killing over 200,000 people.

Physicist J. Robert Oppenheimer convened a conference on the topic of nuclear weapon design
Physicist J. Robert Oppenheimer convened a conference on the topic of nuclear weapon design